In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air-fuel ratio operation in which the combustion chambers contain stratified layers of different air-fuel ratio mixtures. Strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, a three way catalyst optimized for NOx storate, known as a NOx trap or catalyst, is typically coupled downstream of the first three-way catalytic converter.
During lean, rich, and stoichiometric operation, sulfur contained in the fuel can become trapped in the emission control device in the form of SOx. This gradually degrades emission control device capacity for storing NOx, as well as emission control device efficiency. To counteract sulfur effects, various sulfur decontamination methods are available.
One method for determining when to perform a decontamination cycle uses an estimation scheme. In this approach, an amount of SOx stored in the NOx trap is estimated based on operating conditions. For example, an amount of SOx stored is estimated based on driving distance. In another example, the amount of stored SOx is estimated based engine operating conditions. Then, when the estimated amount of stored SOx reaches a predetermined value, the decontamination cycle is performed. These decontamination cycles are capable of removing reversible contamination. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach where permanent, or irreversible, contamination is also present. In particular, depending on a level of permanent contamination, performing a decontamination cycle may not restore enough functionality of the emission control device to warrant operating lean of stoichiometry or to warrant decontamination acts.